Abstract The goal of this project is to identify the downstream effectors of in the process of cilium assembly. Primary cilia are signaling organelles vital for the transduction and integration of key intracellular signaling pathways important for the development and homeostasis of many tissues and organs. Nevertheless, the pathways and cues that govern cilium assembly and disassembly are poorly defined. In previous work, we showed that the IDG-eligible kinase Tau Tubulin Kinase 2 (TTBK2) is a key regulator of early steps of cilium initiation and is also important to maintain the stability of the cilium in post-mitotic cells. Mutations in Ttbk2 also cause a dominantly inherited neurodegenerative disorder, spinocerebellar ataxia type 11 (SCA11). SCA11-associated mutations produce a truncated protein (TTBK2SCA11) that we showed interferes with TTBK2WT in ciliogenesis. However, TTBK2SCA11 does not physically interact with TTBK2WT due to the lack of its C- terminus. Despite the important role of TTBK2 in regulating a key signaling organelle and its links to a human genetic disease, the downstream effectors of TTBK2 are largely undefined. We hypothesize that the dominant interference of TTBK2SCA11 is mediated by its sequestering a subset of key substrates and interacting proteins away from the basal body. Thus, we predict TTBK2SCA11 is an essential tool to use to identify key effectors of TTBK2 in the process of cilium assembly. To test this hypothesis, we will: 1) Compare the protein interactomes of TTBKSCA11 and TTBK2WT using biotin proximity labeling and 2) Define the position of overlapping interactors within the TTBK2 pathway and their role in cilium formation through loss of function experiments and a high throughput endogenous labeling approach. Through this study we expect to identify key effectors of TTBK2 in cilium assembly as well as understand the mechanism of dominant interference of the SCA11-causing truncations, thus providing new insights into a human neurodegenerative disease.